It is known that, for an internal combustion engine having a knock control, a knock-detection method is carried out in which the structure-borne noise picked up by the knock sensor or knock sensors and transformed into an electrical signal is processed in a signal-processing device. The signal-processing device, which, for example, is in the controller of the internal combustion engine, carries out a digital or analog signal conditioning. An essential component of the signal conditioning and of the associated evaluation circuit is a band-pass filter. In this context, the characteristic of the band-pass filter, thus its mid-frequency, quality, etc., must be planned such that the energetic point of concentration of the knock frequencies lies in the passband, while if possible, the frequencies of interference noises should lie outside of the passband. The interference noises are thereby essentially suppressed.
In modern engines, i.e. internal combustion engines, having a plurality of auxiliary systems and actuators, the frequencies of the interference noises may exhibit a strong dependence on rotational speed. Therefore, to be able to ensure good knock detection over the entire rotational-speed range, a speed-dependent filter characteristic is used for such engines or internal combustion engines. A device which carries out the above-described knock detection is described in the publication European Published Patent Application No. 0 576 650.
The device for knock detection known from European Published Patent Application No. 0 576 650 includes in its signal-processing branch, inter alia, a band-pass filter whose mid-frequency is alterable as a function of the engine speed. The mid-frequency in that case is such that the portions of the signal caused by the knocking are filtered as little as possible, while the background or interference signals are filtered out as well as possible. Since the frequencies are able to shift as a function of the rotational speed, a band-pass filter having alterable mid-frequency is used. In the known device, knocking is detected in the customary manner when an integrated measurand, which is formed from knock-typical portions of the signal, differs in a specifiable manner from a value dependent on the background signals.
With a change of the filter characteristic, generally the filtered signal, which represents a measure for the noise of the combustion, changes as well. Since the knock detection is carried out by forming the relationship virkr of the noise, i.e. the so-called knock integral ikr of the instantaneous combustion, and the noise averaged over several preceding combustions of the same cylinder, the so-called reference level rkr, problems may occur upon switching of the filter characteristic. If the value of virkr=ikr/rkr(old) exceeds knock-detection threshold ke, then knocking is detected, i.e., knocking is detected at ikr>rkr(old)*ke.
The reference level is usually calculated recursively during knock-free operation according to the following formula or a similar formula and is described, for example, in German Published Patent Application No. 195 456 49. The following applies:rkr(new)=(1−1/KRFTP)*rkr(old)+1/KRFTP*ikr. 
In this context, KRFTP is designated as the so-called compensation factor.
If a change of the filter characteristic takes place from one combustion to the next, i.e. if, for example, the mid-frequency of the filter is adapted to new or changed circumstances, then the instantaneous combustion noise, i.e. the knock integral ikr, experiences a possibly perceptible change, while the reference level rkr experiences this change, that is to say, is corrected only slowly. If the change of ikr is in particular a distinct increase, a so-called erroneous detection of knocking may occur, that is to say, a non-knocking combustion is mistakenly recognized as knocking. Consequently, changes of the ignition angle are caused which are actually unnecessary. As a result of this first erroneous detection, further erroneous detections may occur, since the knock integral ikr measured for a knocking combustion is not completely taken into account in the reference level rkr, but rather is immediately divided by the factor ke in order to avoid an increase of rkr due to the supposed knocking noise. The calculation is carried out according to the formula:rkr(new)=(1−1/KRFTP)*rkr(old)+1/KRFTP*(ikr/ke)
Conversely, given a perceptible decrease of the combustion noise, the knock detection becomes deaf for a certain time, that is to say, knocking combustions are possibly not detected. Therefore, the change in the filter characteristic leads to a time-limited uncertainty in the knock detection. In addition, each erroneous detection of knocking leads to an unnecessary ignition retard, and therefore to corresponding loss of power and efficiency. On the other hand, the non-detection of knocking combustions causes increased engine wear.